1. Field of the Invention
The present invention relates generally to a printing apparatus for mixing and ejecting a metering medium and a discharge medium and, more particularly, to a printing apparatus which has a means for controlling the spread of ink, diluent, and a mixed solution around a nozzle to form a recorded image of high resolution.
2. Description of the Related Art
In recent years, particularly in offices, documents have increasingly been composed and printed with computers, which is called desk top publishing. It has been recently requested to print not only a character and a figure, but also a colored natural image of a high quality, such as a photograph, along with the character and the figure. Thus, it has been important to reproduce a halftone color.
In addition, an on-demand-type printing apparatus, which discharges a drop of ink solution from a nozzle to a sheet of paper, film, or the like according to a printing signal only when it is necessary for printing, has been used widely in recent years because it is easy to miniaturize the printing apparatus and to lower its cost.
Although various kinds of methods have been proposed for discharging a drop of the ink solution, it is common to use a piezo-electric element or a heating element. The former is a method of discharging a drop of the ink solution by pressurizing the ink with a deformation of a piezo-electric element. The latter is a method of discharging a drop of the ink solution with a pressure of bubbles generated by heating and boiling the ink with a heating element.
In addition, various kinds of methods have been proposed for reproducing the above-described halftone color with the on-demand-type printing apparatus which discharges a drop of the ink solution as described above. The first method is to control the size of a drop of the ink solution by changing a voltage or a pulse length of a voltage pulse provided to the piezo-electric element or the heating element and thus to change the diameter of a printing dot and to produce gradation therewith.
However, in this method too small of a voltage or pulse length provided to the piezo-electric element or the heating element will not discharge the ink. Therefore, this method has a limit to the minimum diameter of a drop of the ink solution, a relatively small number of expressible gradation levels, and a particular difficulty in expressing lower concentrations. Accordingly, this method is not sufficient to print out a natural image of high quality.
The second method is to compose a picture element with a matrix including 4.times.4 dots, for example, without changing the diameter of the dot, and to express the gradation by this matrix, or a dither method. This method can express 17 levels of gradation. However, in this method, for example, if a printing is performed in the same dot density as in the first method, the resolution of the picture in this method is a quarter of that in the first method and coarseness is noticeable. Accordingly, this method is not sufficient to print out a natural image of high quality.
On the other hand, the inventors have proposed a printing apparatus which mixes diluent and ink to discharge a drop of the mixed liquid, changes the concentration of a drop of the discharged ink, and thereby can control the concentration of the printed dot to express the gradation without deteriorating the resolution and to print out a natural image.
According to the above-described concept, there is a print head having a first nozzle into which discharge medium is introduced, and a second nozzle into which metering medium is introduced. The first nozzle is adjacent to the second nozzle. The second nozzle oozes a metered amount of the metering medium toward the first nozzle to mix with the discharge medium in the vicinity of an orifice of the first nozzle. A discharge medium is pushed out from the first nozzle with the discharge medium mixed with the metering medium. The discharge medium and metering medium are thereby discharged in a mixed state in a direction between the faces of the first nozzle and the second nozzle. In this case, one of the above-described metering medium and discharge medium is the ink and the other is a diluent.
In the above-described print head, however, as shown in FIGS. 1 and 2, liquid 104, such as ink, diluent, or a mixed solution or the like, spreads around the orifices of the first nozzle 102 and the second nozzle 103 of the nozzle outlet face 101a where the first nozzle 102 and the second nozzle 103 of the print head 101 are opened. This spreading of the liquid 104 causes a number of problems.
For example, when the liquid 104 adheres to the part around the orifices of the first nozzle 102 and the second nozzle 103, a condition occurs wherein the liquid 104 adheres to an unnecessary part of the print head during printing, thereby causing a worse printing.
In addition, when the liquid 104 adheres to the part around the orifices of the first nozzle 102 and the second nozzle 103, the ink, the diluent, or the mixed solution discharged from each nozzle is displaced in its discharging direction during printing later, which produces a worse printing.
Moreover, when the liquid 104 adheres to the part around the orifices of the first nozzle 102 and the second nozzle 103, there is a strong possibility that it has an effect on a mixing ratio of the ink and the diluent during printing later and, thus, the response to a change of concentration is lost. The gradation of the concentration in the dot, therefore, cannot be correctly reproduced and, therefore, the resolution of the recorded image is lost.
Furthermore, for the print head to reproduce precise shades of dots, the print head must stably mix a specified amount of metering medium with the discharge medium, and detach the mixture from the print head. Accordingly, for the print head with the above constitution to ensure detachment of the mixture comprising the metering medium and the discharge medium from the print head, the print head is provided with a liquid-repellent membrane.
Referring now to FIGS. 3 and 4, for a specified amount of the metering medium to stably mix with the discharge medium in the print head, which is provided with a first nozzle 201 having a round opening to eject the discharge medium and a second nozzle 202 having an elliptic opening to eject the metering medium, the metering medium should not be pushed out equally in all directions from the second nozzle 202, but in a specific direction with respect to the position of the first nozzle 201.
As illustrated in FIG. 4, with the liquid repellent membrane coated on the whole surface around the openings of the nozzles, however, the metering medium 203 ejected from the second nozzle 202 spreads equally in all directions around the opening of the second nozzle 202, as indicated by the arrows A. If the metering medium were allowed to spread equally in all directions, as indicated in FIG. 4, it might happen that the metering medium 203 could not reach the first nozzle 201 from which the discharge medium is ejected. This would obliterate the required mixing of the metering medium and the discharge medium, which would destroy the precise quantification of the volume of the metering medium 203.
To meet such situations, the print head with the above constitution has the first nozzle to eject the discharge medium and the second nozzle to push out the metering medium placed as close together as possible. This arrangement allows a minute to large amount of metering medium 203 to be put to mixing, which then allows reproduction of a wide range of graded tones.
To produce a print head in which the first nozzle 201 and the second nozzle 202 are placed as near as possible, however, it is necessary to improve the positioning precision of the machines responsible for the manufacture of the print head. Thus, stable production of such print heads would be difficult or require a high cost.
In view of above, there is a need for a printing apparatus which suppresses the adherence of ink, diluent and their mixture to the parts around the nozzles, minimizes failures in printing, allows precise reproduction of graded tones, and thereby enables reproduction of high-resolution graphic images, and a method for manufacturing such a printing apparatus.